The present invention relates to an apparatus for reproducing an information signal recorded on an optical disc along spiral or concentric tracks comprising a light source for producing a light beam, an optical system for focusing the light beam onto the optical disc as a light spot, a light detector for receiving the light beam modulated in accordance with the information signal recorded on the disc to produce a reproduced signal, means for rotating the disc at a given revolving speed, and means for returning the light spot on the disc by one track pitch after every revolution of the disc so as to effect a special mode reproduction such as slow motion and still picture mode reproduction.
In a video disc, a video signal is so recorded that successive frame signals are recorded in successive tracks. In order to effect a still picture mode reproduction, the beam spot must be moved backward by one track pitch in a stepwise manner in synchronism with a frame synchronizing signal so as to cause the beam spot to trace the same track repeatedly. For effecting the still picture reproduction accurately, the beam spot should be returned precisely by one track pitch. This may be effected by rotating a movable mirror by a small angle or by shifting an objective lens in a tracking direction substantially perpendicular to an optical axis of the lens as well as to a track direction. However, in practice, it is very difficult to return the beam spot accurately by one track pitch by means of an electro-mechanical mechanism for driving the mirror or lens. That is to say although a given control amount required for moving the beam spot by one track pitch has been previously set, an actual amount of displacement of the beam spot might be fluctuated due to a mechanical tracking property of the driving mechanism. In general, the mechanical tracking property might be subjected to temperature drift and secular variation and thus, the control amount required for moving the spot by one track pitch might be fluctuated. If the driving mechanism is so constructed as to avoid such a fluctuation, the driving mechanism must be complicated and is liable to be expensive to a great extent.